1. Field of the Invention
In general, the invention involves organic light emitting diode (OLED) devices. More specifically, the invention involves luminance uniformity enhancement methods and apparatus for a OLED lighting device used in medical device and similar applications.
2. Related Art
An OLED (Organic Light Emitting Diode) device could be fabricated from either small molecule or polymeric materials. A typical device structure of a polymer light-emitting diode (PLED) consists of an anode (e.g. indium-tin-oxide (ITO)), a hole injection layer (e.g. PEDOT:PSS or polyaniline), an electroluminescent (also light emitting polymer or LEP) layer, and a cathode layer (e.g. barium covered with aluminum). Among the two organic layers, the function of the hole injection layer is to provide efficient hole injection into subsequent layers. In addition, hole injection layer also acts as a buffer layer to smooth the surface of the anode and to provide a better adhesion for the subsequent layer. The function of the electroluminescent layer is to transport both types of carriers and to efficiently emit light of desirable wavelength from electron-hole pair (exciton) recombination. The desired emission wavelength can be conveniently controlled by proper selection of the light-emitting polymers. A well-defined hole transport interlayer between the HIL and LEP can also be used to enhance device efficiency and improve device operational stability. A typical device structure of small molecule-based light-emitting devices consists of an anode, a hole injection layer, a hole transport layer, an electroluminescent layer, an electron transport layer, and a cathode. Among them, the electron transport layer is used to enhance electron transport from the cathode to the emissive layer. The functions of other layers are similar to those in a PLED device.
For some applications such as medical devices, OLED lighting devices usually have to demonstrate sufficiently high luminance uniformity across the whole emitting area. Luminance non-uniformity of less than 10% is often required for a very large emitting area (e.g. 3 cm2). Given these requirements, conventional OLED devices must be specifically re-designed to be successfully used as a light source for medical devices and similar applications.
For instance, compared to the metallic cathode, an ITO anode has much higher resistivity (usually 10-20 Ohm/cm). For relatively large emissive area (e.g., 3 cm2), the voltage drop along the ITO could become significant and result in unacceptable luminance uniformity. A conventional way to reduce the voltage drop is to deposit some high conductivity metal lines on the top of ITO. Although this could solve the voltage drop problem, it usually requires more metal deposition, patterning, and corresponding photolithography processes, causing longer processing time and higher production cost. In addition, because the metal lines are usually opaque, it can also reduce effective emitting area and efficiency.